17 replies to this topic

[antarex]

Is it worth spending money on a dual band filter (OIII and H-ALPHA), having an ALT-AZ mount?
Since, the basic ALT-AZ mount allows limited exposures, and a dual band filter lets little light through.
Would it be possible to use it profitably, even if you are forced to use a high gain and a very high integration time?
Is there anyone with an alt-az mount (without equatorial wedge and guider) who uses dual band filters?

[matt_astro_tx]

I don't have an alt-az mount, but I do have a duoband filter.  I typically shoot 120" exposures with it.  A lot of folks shoot 300" but I find 120 works best for my camera.  You're right I don't know that you'd want to go too short, like 30" you may not get enough signal.

 

How long of an exposure are you able to shoot with your setup before field rotation creeps in?

[daveco2]

The Ha and Oiii light is just as bright with or without filters.  What filters do is exclude all other light that is useless. That’s why filters are very useful under light pollution and moonlight.

 

The problem with Alt-Az mounts is that they cannot take into account field rotation during one exposure or from one exposure to later exposures through the night.

 

if you’re at a high Bortle site, what you could do without filters is take 60 sec exposures for an hour, wait a few hours, then take exposures for another hour.  This will gauge the extent of the problem before investing any money.  See if it’s possible for your stacking software to align all the subs.

Edited by daveco2, 16 April 2025 - 05:52 AM.

[smiller]

Hi,

 

I shoot with not only an Alt/Az mount, but with an unguided mount designed for casual visual astronomy only and at long focal lengths as it’s a stock Goto Dob.  So I’m limited to very short exposures of about 6-8 seconds doing narrowband from B6/B7 skies.  

 

My exposure times are primarily limited to my tracking accuracy and not actually field rotation.  If I had better tracking accuracy I would be able to utilize 15 to 20 second exposures in most cases and only need to lower it when targets are very high in the sky.

 

It needs to be noted though that I maximize my ability to reduce the impact of camera read noise, even when using a dual narrow bound filter, by using a very large pixel camera (ASI2400MC with 5.94um pixels) combined with a low focal ratio (F3.65).  This combined with using a very high gain for minimum read noise minimizes the contribution of read noise to my lights.  

 

But even doing this, when I use my very narrow Optolong L-ultimate filter with 3 nm band passes, I still have about a 15 to 20% read noise contribution, which is significant.  So I materially lose productivity.  If you have a very slow telescope and a very small pixel camera then your situation could be much worse, especially if you are in much darker skies so all those details really matter.

 

My astrobin has examples and you can judge for yourself if the results are good enough to meet your expectations:

 

https://app.astrobin...heJetty#gallery

 

 

If you tell me the following factors, I can help determine how much camera read noise will cost you because you are unguided Alt/Az:

 

1) Your scope focal ratio.

2) Your camera model

3) Which dual band filter (I.e. the bandpass widths)

4) Your Bortle scale

5) Ignoring field rotation, how long can your exposure times be on your system with decent star quality?

Edited by smiller, 16 April 2025 - 12:14 PM.

[antarex]

In the meantime, thanks to everyone.
I will reply in a single message.
In my case too, the big problem is the tracking, even before the field rotation.
I come to the answer to the points:

 

1) Focal ratio of the main telescope F10 1500mm, and I also have a secondary tube F4 120mm that I thought of using with the dual band filter

2) The camera model is an asi224, with small pixels of 3.75 microns

3) I thought of using a dual band filter of 7nm

4) My bortle scale is usually around 7-8

5) It depends on the focal length used. If I use the F10 1500mm tube, the exposure length does not exceed 4s, while with the 120mm I can easily reach 30s

 

So, you can understand the doubt about using a dual band filter. Considering the sky, and the small pixels with the 1500mm F10 tube I would necessarily have to raise the gain a lot and integrate for many many hours. With the 120mm F4 tube, it would probably be better

 

 

@smiller: congratulations for the shots, inevitably with short exposures and narrow band filters, you had to integrate for many hours

Edited by antarex, 16 April 2025 - 06:38 PM.

[Nanotyrannus35]

I use a duoband filter (sv220) with my Asi224 and a 30mm f4 guidescope on an Az-gte. I’ve found that subs of 10-20 seconds with gain of 300 works pretty well on nebulae. I got this picture of the Rosette Nebula in about 10 minutes with that setup

 

Attached Thumbnails

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[smiller]

Yes this will be a bit of a challenge.  I’m away from my computer and preparing for a climb all day tomorrow but I’ll run the numbers in about a day when I’m back.

[antarex]

I use a duoband filter (sv220) with my Asi224 and a 30mm f4 guidescope on an Az-gte. I’ve found that subs of 10-20 seconds with gain of 300 works pretty well on nebulae. I got this picture of the Rosette Nebula in about 10 minutes with that setup

Thank you for the testimony. It is basically my same configuration. Glad to know that it is possible to use a dual band filter, even with this setup.

Have you tried the same subject in the visible? Did you notice much difference in terms of visibility of the nebula, with the sv220?

[Nanotyrannus35]

Thank you for the testimony. It is basically my same configuration. Glad to know that it is possible to use a dual band filter, even with this setup.

Have you tried the same subject in the visible? Did you notice much difference in terms of visibility of the nebula, with the sv220?

I haven’t tried that same nebula with no filters, but I have tried it with the svbony uhc filter, a wide and filter. With the sv220, the stars and light pollution overpower the nebula a whole lot less.

[antarex]

I haven’t tried that same nebula with no filters, but I have tried it with the svbony uhc filter, a wide and filter. With the sv220, the stars and light pollution overpower the nebula a whole lot less.

I also have a svbony UHC filter, happy to know that with 220 filter the result improves. 

You would have an image obtained with svoby UHC, so I immediately understand?

[smiller]

In the meantime, thanks to everyone.
I will reply in a single message.
In my case too, the big problem is the tracking, even before the field rotation.
I come to the answer to the points:

 

1) Focal ratio of the main telescope F10 1500mm, and I also have a secondary tube F4 120mm that I thought of using with the dual band filter

2) The camera model is an asi224, with small pixels of 3.75 microns

3) I thought of using a dual band filter of 7nm

4) My bortle scale is usually around 7-8

5) It depends on the focal length used. If I use the F10 1500mm tube, the exposure length does not exceed 4s, while with the 120mm I can easily reach 30s

 

So, you can understand the doubt about using a dual band filter. Considering the sky, and the small pixels with the 1500mm F10 tube I would necessarily have to raise the gain a lot and integrate for many many hours. With the 120mm F4 tube, it would probably be better

 

 

@smiller: congratulations for the shots, inevitably with short exposures and narrow band filters, you had to integrate for many hours

OK, back from my climb:  

 

 

 

I ran the numbers for the scopes, camera, filter, and bortle scale you noted.  I used Bortle 7 to give a worst case for camera read noise.

 

For the F10 scope, with the 7nm filter and the camera gain at gain 200 (which is what you'll want to use for short exposures and a 7nm filter) I get the following
 

Scope F Ratio: F10

Exposure time: 4 seconds

Filter 7nm/7nm

Camera ASi224MC at gain 200

Bortle 7:

 

Read noise contribution: at 4 seconds is an addition 92.5% over the sky noise.  So you'll need to be imaging for about 4x longer than you would be with very long exposures.

 

Running the same with the F4 scope with 30 seconds then I would run the camera in lower gain as the pixel wells will fill much more quickly and overfill at 30 seconds for many stars.   I selected gain 60 (High conversion gain) with read noise 1.6e.   For that with 30 seconds, read noise contribution is a fairly small 6.8% over sky background. 

 

So with the F4, you're in great shape.  For the F10, 4 seconds is very problematic for 7nm filters as you lose 75% of your imaging productivity.  

 

Now, with no filter (broadband) you are good to go with the F10 even with 4 second exposures.

 

For the F10 with 7nm and B7, every doubling of that exposure time cuts the read noise contribution by nearly half.  

 

One other note, the thermal signal will also cut down on productivity, usually not as much as read noise, but on a warm evening, it can be significant.  I don't have thermal stats for that camera so I can't run the numbers, but if I put in a typical number for an uncooled camera of similar pixel size and assume a typical summer evening, it will also significantly impact productivity in the F10/7nm filter case.  No issues in the winter though.

 

Cheers,

 

Steven

Edited by smiller, 18 April 2025 - 08:19 AM.

[acrh2]

 

So with the F4, you're in great shape.  For the F10, 4 seconds is very problematic for 7nm filters as you lose 75% of your imaging productivity.  

 

 

 

Maybe I am misunderstanding something here. A dual narrowband filter (vs. no filter) will always help with light pollution, the read noise will remain the same. How much a filter will help depends on the proportion of read and shot noise, but judging by the numbers you posted, there would still be a significant benefit from using the filter. The only problems I can see with using a dual narrowband filter and short exposures at f/10 are plate solving and autofocus. 

 

Could you explain in a little more detail what you mean by "lose 75% of your imaging productivity?"

[smiller]

Maybe I am misunderstanding something here. A dual narrowband filter (vs. no filter) will always help with light pollution, the read noise will remain the same. How much a filter will help depends on the proportion of read and shot noise, but judging by the numbers you posted, there would still be a significant benefit from using the filter. The only problems I can see with using a dual narrowband filter and short exposures at f/10 are plate solving and autofocus. 

 

Could you explain in a little more detail what you mean by "lose 75% of your imaging productivity?"

You make a very good point.  When I said “lose 75% of your imaging productivity” I meant in relation to the same situation but longer exposures.

 

But you are absolutely correct that even if read (and thermal) noise dominate, reducing any unwanted source of noise should be beneficial, so I should have also stated the results in terms of productivity gained on a narrowband target when going from 100nm (no filter) to 7nm.  

 

The answer is a bit complicated because a dual narrowband filter not only attenuates unwanted broadband sky noise, but most also cut useful narrowband emissions.  For instance, you often lose not only Sii but also Oiiia and Hb, both of which are about 30% the strength of Oiiib and Ha (from what I’ve been told).  The filter even attenuates the primary emissions of Oiiib and Ha, usually by 10-15% depending on the quality of the filter and F-ratio of the scope.  (I’ll ignore secondary emissions like Nitrogen, Argon, etc… as they are typically only present at material levels in a few planetary nebula from what I understand)

 

If I make a rough assumption that the total loss in narrowband signal is about, ohhh let’s make up a number… say 40% (You lose all Sii, Oiiia, Hb, and perhaps 10% of Ha and Oiiib), then my rough calculation is:

 

Broadband SNR = S/sqrt(100).   = S/10

Narrowband SNR = S*0.6/(2*Sqrt(7))  = S*0.6/5.3 = S/8.8  I used *2 as the read noise roughly doubled the total noise.

 

So you do get a gain, but possibly a modest gain.

 

Perhaps you can check my math… I’m not entirely confident I did it right… I’m a little punchy right now…

Edited by smiller, 17 April 2025 - 08:17 PM.

[matt_astro_tx]

OK, back from my climb:  

 

Lol, that picture is epic.

[antarex]

OK, back from my climb:  

 

Dad and Lenny.jpg

 

 

I ran the numbers for the scopes, camera, filter, and bortle scale you noted.  I used Bortle 7 to give a worst case for camera read noise.

 

For the F10 scope, with the 7nm filter and the camera gain at gain 200 (which is what you'll want to use for short exposures and a 7nm filter) I get the following
 

Scope F Ratio: F10

Exposure time: 4 seconds

Filter 7nm/7nm

Camera ASi224MC at gain 200

Bortle 7:

 

Read noise contribution: at 4 seconds is an addition 92.5% over the sky noise.  So you'll need to be imaging for about 4x longer than you would be with very long exposures.

 

Running the same with the F4 scope with 30 seconds then I would run the camera in lower gain as the pixel wells will fill much more quickly and overfill at 30 seconds for many stars.   I selected gain 60 (High conversion gain) with read noise 1.6e.   For that with 30 seconds, read noise contribution is a fairly small 6.8% over sky background. 

 

So with the F4, you're in great shape.  For the F10, 4 seconds is very problematic for 7nm filters as you lose 75% of your imaging productivity.  

 

Now, with no filter (broadband) you are good to go with the F10 even with 4 second exposures.

 

For the F10 with 7nm and B7, every doubling of that exposure time cuts the read noise contribution by nearly half.  

 

One other note, the thermal signal will also cut down on productivity, usually not as much as read noise, but on a warm evening, it can be significant.  I don't have thermal stats for that camera so I can't run the numbers, but if I put in a typical number for an uncooled camera of similar pixel size and assume a typical summer evening, it will also significantly impact productivity in the F10/7nm filter case.  No issues in the winter though.

 

Cheers,

 

Steven

Thank you, very valuable information. In fact, my idea was to exploit the F4 tube even if it is only a 30/120mm with the dual band filter.

 

Congratulations on the photo!

Edited by antarex, 18 April 2025 - 04:16 AM.

[smiller]

Lol, that picture is epic.

 

 

Thank you, very valuable information. In fact, my idea was to exploit the F4 tube even if it is only a 30/120mm with the dual band filter.

 

Congratulations on the photo!

Ha, thanks!  My daughter’s dog loves to chase us down the mountain.  That was Mount Saint Helens by the way, the one that blew its top in 1980. I was gonna post a picture of us at the top, but I’ll refrain because this is an Astro forum and I’ll probably get scolded.

[matt_astro_tx]

I was gonna post a picture of us at the top, but I’ll refrain because this is an Astro forum and I’ll probably get scolded.

Nah.

[smiller]

Nah.

Ok, here it goes:

 

 

It would be a great place for a scope at night!  Perhaps pack a SeeStar and camp at the top.

 

That’s at the rim over the crater and Mt Rainier in the distance.  We don’t get close to the rim in the winter as it’s just a big overhanging cornice.  You can see Spirit Lake with the logs floating that are from the original eruption.  I never thought the logs would still be floating after 45 years.

Edited by smiller, Yesterday, 08:32 AM.